A conventional XYZθ positioning system is formed by stacking Y-axis assembly onto X-axis assembly, forming two orthogonal axes. Next, the additional Z-axis is stacked onto the X-axis assembly vertically, and then lastly, the θ-axis is mounted on top of the Z-axis. One limitation of such stacking system of the conventional XYZθ positioning system is that the positional accuracy is adversely affected by errors in each axis. The positional errors of the entire system will be the results of all the errors of the different axes coupled together.
A further drawback of such stacking approach in this conventional XYZθ positioning system is that adjustment made to any axis will affect the positioning of other axes that are on top of the particular axis. This means that the alignment of the whole system of the conventional XYZθ positioning system is dependent of the alignment of each axis.
The third disadvantage of the conventional stacked system is the high and large footprint required to stack all the axes. This large footprint makes the design of the system to be spatially not efficient, and even not feasible in most of the machine designs in which spatial constraint is critical. A larger footprint of the XYZθ positioning system in turn affects the footprint of the design of the whole system. For example, U.S. Pat. No. 4,492,356 reveals a design of X-Y table utilizing the stacked-up approach. This design has the Y-axis table being stacked onto the X-axis table, resulting in relatively high footprint of the X-Y table.
Another similar stacking approach positioning system is disclosed in U.S. Pat. No. 6,588,081, in which the θ-axis is stacked on top of the Z-axis. U.S. Pat. No. 6,588,081 also discloses the design of the X-Y stacked table, and a rotary table, θ-axis, is mounted onto the X-Y table. This design also allows for modular X-Y table to be stacked higher up onto the rotary table, forming an even higher footprint system. These stacking systems attribute to the poor footprint efficiency.
With the many limitations in such system, some prior art systems attempt to solve the issues occurred in the stacking system. U.S. Pat. No. 5,040,431 utilizes X-Y stages using air bearing elements and a flat surface plate to decouple the vibration and rolling of the Y stage from the X stage.
Another similar design is revealed in the U.S. Pat. No. 5,228,358, in which the X-Y stage is driven by a linear motor using permanent magnet. The construction of the guidance for these systems is using the air bearings. The number of air bearings used is approximately 14, for guiding the X and Y axis. Such a large number of air bearing units makes the design to be economically inefficient and the air consumption rate is very high.
One approach in resolving the large footprint issue is addressed in U.S. Pat. No. 7,271,879, which uses a decoupled X-Y stage design to obtain a low vertical profile and a moderate footprint size. In U.S. Pat. No. 7,271,879, a moving platform holding the workpiece is floating on the two rigid reference surfaces mounted onto a rigid base. One limitation of such a design is that the calibration and machining efforts are needed for the alignment of the two reference surfaces for parallelism.
U.S. Pat. No. 5,731,641 is a design in providing a lift stage based on wedge design, providing performance in improved acceleration, speed and system bandwidth. However, in terms of the spatial requirement to achieve this lift or Z-axis direction movement, this wedge design is not efficient. The stroke of the lift is a fraction of the horizontal stroke the lift moves, hence accounting for the large footprint in X and Y direction.
U.S. Pat. No. 8,008,815 a discloses a planar stage moving apparatus for a machine comprising: first to fourth linear motors for applying, between a base and a table, a movement force to the table, each linear motor including a stator core on which a coil is wound and which is fixed to the base and a mover core to which permanent magnets are attached and which is fixed to the table; an air bearing unit to provide a repulsive force between the base and the table, to separate the base and the table and thereby permit to move under the influence of magnet fields when currents are applied to the coils of the linear motors, wherein the air bearing unit comprises a plurality of air bearing pads for each of said linear motors, wherein the air bearing pads are provided between the permanent magnets of each of said linear motors; and a linear encoder installed on one side of the table to measure movement of the table, the first and third linear motors being provided between the base and the table on the lower and upper sides of the table respectively to move the table in the X-axis directional movement of the table, the second and fourth linear motors being provided between the base and the table on the right and left sides of the table respectively to move the table in the Y-axis directional movement of the table, the air pads being fixed to a plurality of grooves formed on the lower surface of the table respectively and a plurality of air supply lines formed at intermediate portions of the table to provide a pneumatic pressure to the grooves respectively.
U.S. Pat. No. 7,271,879 relates to planar positioning system, comprising: a rigid base; first and second actuator means each having a fixed portion directly anchored to said rigid base, the first and second actuator means each having a respective moveable portion that is linearly moveable relative to the rigid base, wherein the first and second actuator moveable portions are restricted to movement in respective first and second orthogonal linear dimensions; a flat reference surface mounted to said base; a moveable platform for holding a workpiece, the moveable platform being supported for planar movement over said reference surface; a first coupling between the first actuator moveable portion and the moveable platform for effecting movement of the platform in the first linear dimension, the first coupling also serving to guide the platform in the second linear dimension; and a second coupling between the second actuator moveable portion and the moveable platform for effecting movement of the platform in the second linear dimension, the second coupling also serving to guide the platform in the first linear dimension.
U.S. Pat. No. 7,257,902 discloses a stage device comprising: a base; a stage carrying a movable body and being moved over the base; a planar motor driving the stage; an air bearing acting to lift the stage over the base; a scale part disposed on the base to include an angle grating which has an angle-related characteristic varied in a two-dimensional direction in accordance with a known function; and at least one two-dimensional angle sensor disposed on the stage so that the at least one two-dimensional angle sensor emits a light beam to the angle grating of the scale part and detects a two-dimensional angle of a light beam reflected from the scale part.